What are possible downsides of practice where I download a bitcoin wallet on my phone, write down the seed and the address, purge the app into nonexistence and continue my happy life, sharing my address, using it like, for example, a savings account?

3 Answers
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If you only generate a limited set of addresses before you delete your wallet, it is likely you will run out of fresh addresses to receive funds on, meaning multiple spendable transactions will be linked together via the receiving address. This is a privacy disadvantage.

Secondly, your mnemonic key phrase back-up follows an off-chain deterministic key derivation standard (BIP32/43/44), and even if all consensus rules do not chain, there is no guarantee that wallets continue to implement the same key generation method that they currently do (depending on your timeframe). If you cannot use a wallet application with the same key derivation standard later, you will have to derive your used keys yourself using the BIP32/43/44 standards.

This is a good answer. I liked the part about privacy. Can you please explain more the second part? I'm not sure I follow.
– Igor V.Jan 25 at 9:43

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Your mnemonic key phrase is used to derive the actual private keys and addresses which u receive find on. This derivation path happens entirely in your wallet and hs nothing to do with the Bitcoin network, it is simply a standard method adopted by wallet makers. If your wallet maker no longer supports this and updates to sth else, it could be that your mnemonic phrase results in different key/addresses in the future. It is unlikely that a wallet wouldnt continue to support older mnemonic phrases in the future, but there are no guarantees. In that case u would have to derive your keys manually.
– James C.Jan 25 at 10:12

If this reply answered your question entirely (and only if it did), a checkmark would be kindly appreciated!
– James C.Jan 27 at 14:20

I will gladly do so if no other, better, answer comes up in near future.
– Igor V.Jan 27 at 14:29

@IgorV. i suggest writing down the name of the wallet software you used and the version number along with the seed backup. this will help you restore your wallet in future.
– AbdussamadFeb 8 at 12:27

If security is your primary concern, the best practice would be to generate all private keys offline. (Ideally you would do it on a device that will never be connected to the internet or manually e.g., with dice.) If you generate the keys on your phone, even if you delete the app, you would need to make sure nothing slips out before you delete the app, and that you completely wipe all traces. A rogue program running on your phone could compromise you when you generate your private key.

You don't even need to install an app for that purpose. There are a number of tools available online which allow you to generate (both randomly and not-so-randomly) BIP39 seed words (12-24 word variants) and list your resulting private and public keys, as well as addresses. The Ian Coleman BIP39 generator is a great example. It can be saved and used offline, and if you don't trust his RNG implementation, you can generate your own random number(s) using a variety of options, including just rolling a (presumably fair) 6-sided die multiple times, shuffling and drawing cards, or plugging in hexadecimal, binary, or base-10 numbers generated from the RNG of your choice.